1. Field of the Invention
The present invention relates to a chair in which a shape of a backrest surface can be three-dimensionally changed while following a motion of a body of a person sitting thereon.
2. Description of the Related Art
There has been known a chair in which a backrest surface is structured by bridging an upholstery member between a pair of frame elements formed in both right and left sides. For example, Japanese Unexamined Patent Publication No. 2002-119375 and Japanese Unexamined Patent Publication No. 2002-119373 disclose a chair in which an upper portion of a backrest surface is supported by an upper frame element, a lower portion of the backrest surface is supported by a lower frame element, and the backrest surface can be formed in correspondence to a body type and a backbone shape of a sitting person through a rotating operation of the upper frame element and a rotating operation around a horizontal axis of the lower frame element.
In this case, as everyday concerns, a person frequently looks around, reaches out a hand or twist himself or herself while sitting on a chair. The motion is executed for various purposes such as turning to a calling person, taking up a goods at the back, easing the stiffness by stretch or the like. However, the conventional chair does not take into consideration following the motion of the sitting person.
More specifically speaking, when looking around, reaching out a hand or twisting himself or herself while sitting on the chair, a left side of the body or a right side of the body is relatively turned down backward. However, in the existing chair such as the example mentioned above, since both the right and left frame elements are rigidly coupled to each other via a lateral bridging member, and these frame elements are integrally moved backward and forward, it is impossible to make only a left side portion or a right side portion of the backrest surface displace backward. Accordingly, it is impossible to deform the backrest surface while following the motion such as the sitting person looks around or the like, and a motion of the sitting person is limited.